The present invention generally relates to lighting systems and related technologies. More particularly, this invention relates to fluorescent lamps and coating systems utilized by fluorescent lamps to generate visible light.
Fluorescent lamps have been in use and commercialization since the 1930s. More recently, both consumers and producers have voiced increased concerns for energy efficiency and environmental impact of products, spanning all industries, including the lighting industry. As such, fluorescent lights have seen an increase in usage due to their increased energy efficiency when compared to conventional incandescent lights. Fluorescent lights see a great deal of competition from light-emitting diode (LED) lights, due to a potential for greater efficiency and luminosity of LEDs. Significant effort and research have been made in the interest of improving fluorescent light lumen output without increasing power requirements or significantly increasing material costs.
A nonlimiting example of a fluorescent lamp 10 is schematically represented in FIG. 1. The lamp 10 is represented as having a sealed glass tube comprising of a glass envelope or shell 12 enclosing an interior chamber 14. The chamber 14 is preferably at very low pressure, for example, around 0.3% of atmospheric pressure, and contains a gas mixture having at least one constituent that can be ionized to generate radiation that includes ultraviolet (UV) wavelengths. According to the current state of the art, such a gas mixture includes one or more inert gases (for example, argon) or a mixture of one or more inert gases and other gases at a low pressure, along with a small quantity of mercury vapor. Electrodes 16 inside the chamber 14 are electrically connected to electrical contact pins 18 that extend from oppositely-disposed bases 20 of the lamp 10. When the contact pins 18 are connected to a power source, the applied voltage causes current to flow through the electrodes 16 and electrons to migrate from one electrode 16 to the other electrode 16 at the other end of the chamber 14. In the process, this energy converts a small amount of the mercury to a charged (ionized) gaseous (vapor) state. The electrons and charged gas molecules move through the chamber 14, occasionally colliding with and exciting the gaseous mercury molecules, raising the energy level of the electrons in the mercury atoms. In order to return to their original energy level, the electrons release photons.
Due to the arrangement of electrons in mercury atoms, most of the photons released by these electrons are in the ultraviolet (UV) wavelengths. This is not visible light, and as such for the lamp 10 to emit visible light these photons must be converted to a visible light wavelength. Such a conversion can be performed by a coating 22 disposed at the interior surface of the glass shell 12. Such a coating 22 often contains phosphor powders and may be separated from the glass envelope or shell 12 by a UV-reflecting barrier layer 24 of, for example, alumina (Al2O3) or a halophosphor (such as a calcium halophosphate phosphor). The UV wavelengths emitted by the ionized mercury vapor are absorbed by the phosphor composition within the coating 22, resulting in excitation of the phosphor composition to produce visible light that is emitted through the glass shell 12. More particularly, when electrons of the phosphor atoms are struck by photons, the electrons become excited to a higher energy level and emit a photon to return to their original energy level. The emitted photon has less energy than the impinging photon and is in the visible light spectrum to provide the lighting function of the lamp 10. The color and luminosity of the lamp 10 are largely the result of the phosphor or phosphors used in the coating 22.
A great deal of effort has been made to increase the luminosity of fluorescent lamps without increasing the power requirements, thereby increasing their efficiencies. Currently, the most effective solution to increase luminosity involves the use of rare earth-containing phosphor compositions, including rare earth compounds and transition metal compounds that serve as host materials doped with one or more rare earth activators that, for example, prolong emission time. As nonlimiting examples, phosphor compositions have been employed that contain a controlled mixture of europium-activated barium magnesium aluminate phosphor (BAM, which may be considered as having formula BaMg2Al16O27:Eu2+) as a blue-emitting phosphor, cerium- and terbium-coactivated lanthanum phosphate phosphor (LaPO4:Ce3+, Tb3+; LAP) as a green-emitting phosphor, and europium-activated yttrium oxide phosphor (Y2O3:Eu3+; YEO) as a red-emitting phosphor, mixed in appropriate ratios. As known in the art, the term “activated” refers to the effect that doping with europium, cerium, terbium, and other dopants have with respect to the luminescence of a phosphor.
Rare earth elements are generally considered to include seventeen chemical elements, namely, the fifteen lanthanides, scandium, and yttrium. Despite their name, rare earth elements are fairly abundant. Nevertheless, rare earth elements are becoming increasingly prohibitive to procure and use. Demand for these materials, which are also used in medical technology, wind turbines, hybrid automobiles, TVs, smart phones, and computers, as well as other industries, has significantly increased as these technologies have grown. In addition, rare earth materials may occur as mixtures of various rare earth compounds and in low concentrations. These compounds must be mined, concentrated into oxides, and finally separated into individual rare earth elements or compounds suitable for the intended application. Finally, the majority of the world's rare earth reserves are currently believed to be in limited locations, leading to concerns over pricing and supply stability. As such, the use of rare earth elements has or may become cost-prohibitive, and effective alternative solutions are being sought.
One approach to maximizing the visible light output of a fluorescent lamp is through the utilization of a scattering agent. As noted above, fluorescent lamps have incorporated UV-reflecting barrier layers (24 in FIG. 1) that contain, for example, alumina. Alumina powders have been particularly used as a scattering agent in fluorescent lamps for several decades. While alumina is a very effective scattering agent, it also traps water molecules in the coating. In addition, excessive amounts of alumina powder may negatively impact the effectiveness of the phosphor coating, and may have an adverse effect on the performance of the lamp as a whole. Consequently, while alumina is an effective scattering agent for use in fluorescent lamps, there are limits to its performance-enhancing abilities, and effective alternative solutions have been sought.